Saw the Zemeckis version of Beowulf over the weekend. If you can see it in 3-D, do so. Otherwise, I'd wait for the rental. Here's the Chronicle review, which points out the seizmic changes from the original. The Weekly Standard review that I read was even more critical of this modern retelling.
http://chronicle.com/weekly/v54/i15/15b00101.htm
A somewhat different version of Beowulf can be seen in this movie, though it shares the modern sensibility that Zemeckis takes of "misunderstood monsters" and "guilt-ridden" heroes who lack appropriate humility.
http://www.netflix.com/Movie/Beowulf_Grendel/70052021?trkid=189530&strkid=1225017604_1_0